In many industrial environments, fire is a constant and almost unavoidable hazard. This is particularly true of the oil and gas industry. Where highly flammable liquids are being handled under high pressures, it is vitally important that, once a local outbreak of a fire does occur, all internal packing elements maintain pressure tight seals lest a local mishap assume uncontrollable proportions.
A wide variety of packing elements designed to maintain a pressure differential between telescoping parts in a connection are generally known in the art. Such materials are most commonly selected, however, on the basis of such features or properties as compressibility, elasticity, chemical resistivity, and lubricity. Some of the most common packing materials, such as gasket paper, cork composition, or sheet rubber are strictly limited in their application to low temperature environments. A number of special, high temperature resistant packing materials are known in the art, as for example, asbestos which is widely used in the form of asbestos mats or sheets or as filling material between corrugated sheet metal, metal jackets or spirally wound steel strips. Even though these materials or combinations of materials do not burn, char, or disintegrate under high temperature conditions, maintaining a pressure tight seal using such materials under such conditions still presents a difficult engineering problem. In most cases, a sizable temperature differential exists between the two surfaces that are to be sealed, especially where high temperatures are locally confined and intermittent in nature, e.g., where a fire breaks out within or around the telescoping machine elements containing the packing between them. Frequently, the material dimensions of the elements joined or separated by the seal are quite different and will therefore result in different equilibrium temperatures after heating. In addition, even when temperature equilibrium is maintained across the interface, different materials employed side by side will undergo different thermal expansions and thereby cause leakage or loss of sealing effect. If, for example, the outer element is both longer in axial dimension and hotter in temperature, its greater axial expansion tends to relieve any load which had been supplied by it to actuate the seal, thereby permitting leakage to result. The differing radial expansion between the (hotter) outer element and the (cooler) inner member will also result in a radial gap which destroys the seal or permits it to extrude into the gap. Since prior art packings have generally employed packing materials of thermal expansion coefficients substantially different from those most commonly used for the stuffing box, e.g., steel, both types of sealing breakdowns are experienced under high temperature differential conditions. It would, therefore, be desirable to employ a packing material having a thermal expansion coefficient such that it will expand approximately as fast as, or slightly faster than, the elements to be sealed when the elements are exposed to a high temperature environment. It would also be desirable to employ an applicator which prevents extrusion of the packing material while maintaining a fairly constant sealing load upon the packing material and which applies a load greater than the fluid pressure being sealed off between the elements.
Graphite has long been known to have some of the properties generally desired for high temperature packing materials. It must, however, frequently be combined with other materials, which, in turn, negatively influence the thermal properties before its mechanical properties can satisfy the design demands of high pressure seals. One of the substances not sharing this common shortcoming is "Grafoil" described in the publications "Grafoil--Ribbon-Pack, Universal Flexible Graphite Packing for Pumps and Valves" by F. W. Russell (Precision Products) Ltd. of Great Runmow, Essex, England, and "Grafoil Brand Packing" by Crane Packing Company of Morton Grove, Ill. and incorporated herein by reference.
Grafoil is an all-graphite packing material manufactured by DuPont containing no resin binders or inorganic fillers. It has the chemical inertness and lubricity typical of pure graphite but, unlike conventional graphite, has highly directional values for thermal conductivity, thermal expansion, and electrical resistivity. Grafoil is a material which will withstand extreme temperatures over 2000.degree. F., is self-lubricating, will not vulcanize or bond to metal surfaces, exhibits no embrittlement and possesses high thermal conductivity. Its highly directional thermal properties can be readily controlled by the manner in which the material is wrapped. A loose wrap produces packing having the highest thermal expansion in the axial direction. A tight wrap produces the reverse, i.e., highest thermal conductivity axially, and highest thermal expansion radially. A medium wrap will exhibit nearly isotropic thermal conductivity and expansion.
Grafoil has been used in various applications, many of which are described in the above Grafoil publications. Grafoil packing has been used in fire safe controls because the packing will not deteriorate under high temperatures and sealing is maintained after cool down. Grafoil has also successfully been employed in boiler feed pumps, centrifugal, reciprocating and rotary pumps, and for valve stems. It has, however, been limited in application since Grafoil is susceptible to extrusion.
In the oil and gas industry, where extremely high downhole pressures are encountered and must be sealed against, no successful mechanical combination of packing materials has been found which would permit the thermally desirable properties of graphite to be employed under the necessary high pressure conditions. These deficiencies in the prior art are overcome by the present invention.